Cellular radio communication systems, in particular mobile radio systems, are divided into synchronized and non-synchronized radio communication systems.
In the former, base stations of adjacent radio cells are mutually synchronized in terms of time and/or frequency. For synchronizing, in particular for time synchronizing, GPS receivers are employed for instance on the part of the base stations, or base stations are synchronized with each other by synchronizing signals requiring to be exchanged at high cost. Since, moreover, radio transmission resources are occupied during the transmission of synchronizing signals, the resources will no longer be available for transmitting chargeable useful subscriber data (payload).
In non-synchronized radio communication systems, base stations of adjacent radio cells are not synchronized with each other.
As a representative instance of a mobile radio system, FIG. 3 shows a cellular radio communication system according to the related art.
Three adjacent radio cells FZ1 to FZ3 each have an assigned base station BTS01 to BTS03. Each of the base stations BTS01 to BTS03 provisions a number of the mobile stations T01 to T012 assigned to the respective radio cell FZ1 to FZ3, with a total of four carrier frequencies f9 to f12 being assigned by a “frequency reuse” planning method to a first base station BTS01 of a first radio cell FZ1, a total of four carrier frequencies f1 to f4 being assigned thereby to a second base station BTS02 of a second radio cell FZ2, and a total of four carrier frequencies f5 to f8 being assigned thereby to a third base station BTS03 of a third radio cell FZ3 exclusively for data transmission.
In a connection direction referred to as the “downlink” DL from the base station to the mobile station, each of the carrier frequencies f1 to f12 has seven timeslots TS1 to TS7 as radio transmission resources, while in a connection direction referred to as the “uplink” UL from the mobile station to the base station each of the carrier frequencies f1 to f12 has five timeslots TS1 to TS5 as radio transmission resources. Free, unused timeslots are shown by way of example for the carrier frequencies f2, f7, and f11 and designated by the letter “F”.
The use of what are termed “orthogonal frequency division multiplexing” (“OFDM” for short) transmission technologies is gaining increasing significance in particular for mobile radio networks of cellular design because new services such as, for instance, transmitting video at fast data rates can be transmitted cost-efficiently with the aid of the technologies.
In radio communication systems, in particular in an OFDM radio communication system, the necessary multiple use of carrier frequencies in adjacent radio cells gives rise to what is termed “co-channel interference”, which can be reduced with the aid of what is termed a “frequency reuse” planning method.
FIG. 4 shows, referred to FIG. 3, a synchronizing situation of the radio cells FZ1 to FZ3 that corresponds to the related art.
It is assumed below that the system is a time-synchronized radio communication system whose adjacent radio cells FZ1 to FZ3 have a “frequency reuse” factor of one, which is to say the radio cells FZ1 to FZ3 employ the same carrier frequencies.
Each base station BTS01 to BTS03 and the mobile stations T01 to T012 assigned to each base station BTS01 to BTS03 have in each case a base-station-specific carrier frequency deviation Delta01 to Delta03 which deviates from a predefined value MIT and is plotted vertically. The carrier frequency deviation Delta01 to Delta03 is due in each of the individual base stations BTS01 to BTS03 to electrical components of the respective base station, for example to base-station-specific local oscillators.
In particular when OFDM radio transmission technologies are used in a synchronous radio communication system, the fast data rates employed necessitate extremely accurate synchronizing which, however, can only be implemented at very high cost.